Thanks to Rob on that other forum who wrote to and received a ruling from Cuban Customs:

I today received this advice (English version via Google translation) from Cuba after writing to Customs for advice:
We acknowledge receipt of your message, in this respect we can inform you:
The importation into the country of Satellite Positioning Systems for geodetic and hydrographic purposes will require prior authorization from the National Office of Hydrography and Geodesy of the Ministry of the Revolutionary Armed Forces.
The importation of Satellite Positioning System (SPS) equipment for location purposes, contemplated within the personalized applications of cell phones, watches, cameras and videos and other portable devices that accompany the passengers, will be authorized without the collection Of customs duties when they classify as personal effects.
Sincerely,
Legal Adviser
Public attention
The advice was actually in Spanish and anyone wanting to see what I asked and what was replied in SPANISH is welcome to send me a PM.

Thanks Spunky
Of course there is never any guarantee as to what Customs in any country might choose to “prohibit”, but the bottom line here is that a vehicle navigation device (they are not really a true GPS) is used for “personalized applications”, is a “portable device” and is one’s"personal effects".
I hope everyone on this forum knows how to correctly complete the Cuban Customs Declaration, because I didn’t until a week ago when I understood how the Spanish interpretation of “complete” and “fill in” needed to be specifically interpreted, and even then others on Tripadvisor don’t necessarily agree.

Thanks for your efforts, Rob.
Every year we watch fellow passengers listing all of their cameras and phones, pads, iPods and everything else on their declaration. Knowing that you don’t list items for personal use, just what you are leaving behind is a shock to some. Not having to pass in the form or not having someone at the door to collect, usually has a few people on the bus wondering if they pass them in at the hotels.